The present invention relates to a process for the preparation of optically active alcohols by catalytic reduction of the corresponding optically active carboxylic acids with hydrogen.
It is already known that optically active alcohols can be prepared by catalytic hydrogenation of the corresponding carboxylic acids on ruthenium catalysts (see EP-A 717 023). The suitable reaction conditions stated therein are temperatures in the range from 50 to 150xc2x0 C. and pressures in the range from 5 to 250 bar.
In the preparation of S-1,2-propanediol (according to an older nomenclature also to be referred to as L-1,2-propanediol), this gave enantiomeric excesses of more than 97% at a yield of at most 88% and a reaction temperature of 80xc2x0 C. In the corresponding hydrogenation of S-malic acid (according to an older nomenclature also to be referred to as L-malic acid), this gave S-1,2,4-butanetriol in a purity of 97% at 80xc2x0 C. A value for the enantiomeric excess was not given.
The enantiomeric excesses of the alcohols prepared in this manner do not yet entirely meet the high requirements for intermediates of active compounds. Moreover, the chemical yields that can be obtained and the reaction times required are not yet entirely satisfactory.
There is therefore still a need for a process for preparing optically active alcohols from the corresponding carboxylic acids in which the overall combination of the enantiomeric excess that can be obtained, the chemical yield that can be obtained and the reaction time required is more favourable than before.
A process for the preparation of optically active alcohols from optically active carboxylic acids has now been found which is characterized in that optically active carboxylic acids are reduced with hydrogen in the presence of a catalyst which, in addition to ruthenium, comprises at least one further metal or transition metal having atomic numbers in the range from 23 to 42.
Examples of optically active carboxylic acids which can be used in the process according to the invention are those of the formula (I): 
in which
m is 1, 2 or 3,
n is zero or an integer from 1 to 5 and
Rxe2x80x2 is a monovalent radical selected from the group comprising linear and branched C1-C12-alkyl, C7-C12-aralkyl, C6-C10-aryl and C1-C12-alkoxy radicals or is hydroxyl or halogen, and
if m=1
R is a monovalent radical selected from the group comprising linear and branched C1-C12-alkyl, C7-C12-aralkyl, C6-C10-aryl and C1-C12-alkoxy radicals or is a halogen or hydroxyl radical which is different from Rxe2x80x2,
if m=2
R is absent or is a divalent radical selected from the group comprising linear and branched C1-C12-alkyl and C7-C12-aralkyl radicals, and
if m=3
R is a trivalent radical selected from the group comprising linear and branched C1-C12-alkyl and C7-C12-aralkyl radicals,
giving optically active alcohols of the formula (II): 
in which
m, n, Rxe2x80x2 and R are as defined for the formula (I).
Preferably, in the formulae (I) and (II),
m is 1 or 2,
n is zero, 1 or 2 and
Rxe2x80x2 is linear or branched C1-C4-alkyl, benzyl, hydroxyl, fluorine, chlorine or bromine, and
R is different from Rxe2x80x2 and is linear or branched C1-C4-alkyl, benzyl, hydroxyl, fluorine or chlorine.
A further preference is for one of the radicals Rxe2x80x2 and R to be hydroxyl.
If m is 2 or 3, i.e. there are 2 or 3 molecular moieties CH(Rxe2x80x2)xe2x80x94(CH2)nxe2x80x94COOH present in the starting material, these molecules can be identical or different in respect of the definitions of n and Rxe2x80x2.
If Rxe2x80x2 and R are alkyl, aralkyl, aryl and/or alkoxy radicals, these can optionally be substituted, e.g. by halogen, hydroxyl, C1-C4-alkoxy, thiol, amino and/or C1-C4-alkylamino radicals. Rxe2x80x2 and/or R preferably contain fewer than four such substituents. Reductions and/or cleavage reactions may also take place at such substituents.
If Rxe2x80x2 and R are alkyl, aralkyl, aryl or alkoxy radicals, these can optionally contain heteroatoms, e.g. oxygen, sulphur and/or nitrogen atoms, in the alkyl chain and/or in the aryl moiety. Preferably, fewer than 3 such heteroatoms are present.
If Rxe2x80x2 and R are aralkyl or aryl radicals, these can also be partially or fully hydrogenated.
If a plurality of COOH groups is present in a compound of the formula (I), it is possible, after the reduction of one of these COOH groups to the CH2OH groups, for lactone formation with one of the remaining COOH groups to occur.
Particularly preferably, the acid used in the process according to the invention is optically active lactic acid, optically active tartaric acid, optically active 2-chloropropionic acid, optically active 4-chloro-3-hydroxybutyric acid or optically active malic acid, giving optically active propane-1,2-diol, optically active 1,2,3,4-butanetetraol, optically active 2-chloropropanol, optically active 4-chloro-1,3-butanediol or optically active 1,2,4-butanetriol respectively.
Suitable catalysts for the process according to the invention are, for example, bimetallic ruthenium/metal X catalysts and trimetallic ruthenium/metal X/metal Y catalysts, all of which can be used as such or when applied to a support, where X and Y in each case represent a metal having an atomic number in the range from 23 to 82. The catalysts can comprise ruthenium and the metals X and, if appropriate, the metals Y in different forms, for example in elemental form, in colloidal form, in the form of compounds of ruthenium and the metals X or of ruthenium and the metals X and Y or in the form of an intermetallic compound of ruthenium and the metal X and, if appropriate, the metal Y. If the catalysts are employed unsupported, they can be present, for example, in colloidal form or as a finely dispersed solid. Examples of catalysts are finely dispersed ruthenium/rhenium, ruthenium/osmium, ruthenium/iron, ruthenium/cobalt, ruthenium/rhodium, ruthenium/palladium, ruthenium/platinum, ruthenium/copper, ruthenium/zinc, ruthenium/silver, ruthenium/tin, ruthenium/germanium, ruthenium/gallium, ruthenium/lead, ruthenium/rhenium/copper, ruthenium/rhenium/silver and ruthenium/rhenium/tin particles, for example in metallic form or in the form of their oxides, hydroxides, halides, nitrates, carboxylates or acetylacetonates.
Suitable supports are, for example, carbons, carbon blacks, graphites, aluminium oxides, silicon dioxides, silicates, zeolites and aluminas. Supported catalysts can comprise, for example, from 1 to 50% by weight of metal, in elemental form or in the form of compounds.
The catalysts to be employed can, if appropriate, be modified by a treatment with sulphur compounds, for example with thioether.
Preference is given to catalysts comprising unsupported rhenium and ruthenium and having a high specific surface, for example of from 50 to 150 m2/g. Such catalysts can be prepared, for example, by reductively depositing, by action of hydrogen, rhenium from a rhenium solution on a ruthenium oxide hydrate with a high surface (for example from 50 to 300 m2/g). What is obtained is a bimetallic catalyst having a high surface, where the two metals are in intimate contact. In principle, a second metal can be deposited during the preparation of the catalyst or in situ during the hydrogenation reaction.
Based on 1 mol of optically active carboxylic acid employed, it is possible to use as catalyst e.g. 0.1 to 30 g of metal or metal compounds or 1 to 100 g of supported catalysts containing metal or metal compounds.
The process according to the invention is in general cared out in the presence of a solvent for the optically active carboxylic acids and the optically active alcohols. Examples of suitable solvents are water, water-miscible organic solvents and mixtures of the two. Water-miscible solvents which may be mentioned are lower alcohols and water-miscible ethers. Preferred solvents are water and mixtures containing water and lower alcohols or tetrahydrofuran.
The process according to the invention can be carried out, for example, at temperatures in the range from 0 to 150xc2x0 C. and pressures in the range from 5 to 300 bar. Preference is given to temperatures from 40 to 130xc2x0 C. and pressures from 50 to 250 bar. Particular preference is given to temperatures from 30 to 80xc2x0 C. and pressures from 150 to 250 bar.
To work up the reaction mixture, it is possible for example firstly to cool it and separate off the catalyst, e.g. by filtration, and then to neutralize the product solution, for example with aqueous sodium hydroxide solution, to remove the highly volatile components present (in general solvent and water of reaction) by distillation, if appropriate under reduced pressure, and fractionate the residue under vacuum. The catalyst which has been separated off can be re-used, as can the solvent. The process according to the invention can be carried out continuously, semi-continuously or batchwise. Suitable reactors are, for example, stirred tanks and trickle-bed reactors. The process is advantageously carried out as a batch feed process, where the catalyst is initially charged in the solvent and the acid is pumped in in the amounts consumed during the hydrogenation. Owing to this, it is possible to keep the acid concentration in the reactor at a low level, which has a positive effect on the operating life of the catalyst and on the yield and reduces the corrosiveness of the reaction medium.
When optically active malic acid is reduced by the process according to the invention, the reaction mixture obtained can be converted directly, i.e. without prior separation of byproducts, into the optically active 3-hydroxytetrahydrofuran, for example by firstly cooling it, separating off the catalyst, removing the highly volatile components by distillation, if appropriate under slightly reduced pressure, adding p-toluenesulphonic acid and carrying out fractional distillation under reduced pressure.
In the reduction according to the invention of optically active malic acid, in addition to optically active 1,2,4-butanetriol, optically active 3,4-dihydroxybutyric acid is generally obtained, some of which lactonizes even under the reaction conditions to give the optically active 3-hydroxybutyrolactone. This is in particular the case if the reaction is terminated at from 50 to 60% of the hydrogen consumption which would be required for the complete reduction to 1,2,4-butanetriol, the highly volatile components are removed, the residue is neutralized, for example with aqueous sodium hydroxide solution, and the 1,2,4-butanetriol is separated off, for example by distillation. After the 3,4-dihydroxybutyric acid has been liberated and lactonized to the optically active 3-hydroxybutyrolactone by addition of acid, for example trifluoroacetic acid (see Chem. Lett. (1984), 1389), the optically active 3-hydroxybutyrolactone can be separated off by distillation.
Surprisingly, catalysts to be used according to the invention, in particular those comprising ruthenium and rhenium, have a considerably higher performance in the hydrogenation of optically active carboxylic acids than a monometallic ruthenium catalyst. Using the process according to the invention, it is possible, in a simple manner, to produce, even on an industrial scale, optically active alcohols having a higher purity, a higher enantiomeric excess, in a higher yield, at lower temperatures and/or with shorter reaction times than when the known hydrogenation processes are used. In particular, the overall combination of the enantiomeric excess that can be obtained, the chemical yield that can be obtained and the reaction time required is more favourable than in the prior art.